Anatomical descriptions of the organizations of auditory afferent and efferent systems have recently undergone substantial revisions. The new picture is so different from previous conceptions that it cannot fail to change the nature of functional models proposed for efferent control of auditory inputs. This new knowledge, however, has yet to be fully incorporated into the design of physiological experiments on the efferent auditory system. The aim of the proposed research is to build upon our recent work which has revealed a richly detailed organization of the auditory efferent system. The experiments will involve recording from efferent fibers, systematically describing their responses to sound, and tracing labeled, physiologically-identified efferent fibers to their origins and destinations. Other experiments will be done to determine the effects on single auditory-nerve fibers produced by stimulating the medial olivocochlear fibers. These experiments are designed to fill large gaps in our knowledge of the physiology of auditory efferents using methods which are well established in our laboratory. The study of the auditory efferent system has general implications for work on centrifugal pathways in other sensory systems and the proposed experiments should substantially increase our knowledge of this little understood aspect of neurophysiology.